In the field of wire rod secondary processing, wire rod after hot rolling is generally subjected to a series of treatments and processes such as heat treatment, acid pickling, coating treatment, and cold drawing so that tertiary processing can be easily performed to form parts.
For acid pickling of wire rod, batch processes are widely used, in which coiled wire rod is immersed as it is in an acid pickling solution. In a batch-type acid pickling process, coiled wire rod is generally put on a C type hook for carrying and transferring when immersed in a pickling tank. A series of acid pickling and coating processes generally include acid pickling with a solution of an acid such as hydrochloric acid or sulfuric acid in an acid pickling line, subsequent water washing, neutralization, and/or coating treatment, which are sequentially performed.
Acid pickling solutions used in acid pickling typically contain sulfuric acid or hydrochloric acid. When sulfuric acid is used, the sulfuric acid solution reaches the surface of the wire rod material through cracks in scale to dissolve the metal and produce hydrogen, so that the scale is removed from the surface of the wire rod. When hydrochloric acid is used in acid pickling, the hydrochloric acid solution reaches the surface of the material through cracks in scale to dissolve the metal and the scale.
In acid pickling solutions, the main chemical reactions occurring on the surface of wire rod are represented by the formula below.
(Dissolution of Steel: Anodic Reaction)Fe→Fe2++2e  (1)(Hydrogen Production: Cathodic Reaction)2H++2e→H2  (2)Fe2O3+6H++2e→2Fe2++3H2O  (3)Fe3O4+8H++2e→3Fe2++4H2O  (4)
When sulfuric acid is used, reactions (1) and (2) mainly occur. When hydrochloric acid is used, reactions (1), (2), (3), and (4) occur. In reaction (1), non-uniform material erosion occurs, corresponding to the metal structure of steel, which can affect the surface roughness. After the pickling, the acid solution is washed away with water. In this process, however, brown or black smut can remain on the surface of steel to turn it to a dark color.
As used herein, the term “scale” refers to oxide produced on the surface of wire rod when the wire rod is rolled and annealed. The term “descaling” refers to removing the scale. The term “smut” is a generic name for black and brown substances that adhere to the surface of steel when scale and wire rod materials are dissolved by acid pickling. Carbon, Si, Cr, Mn, and other components contained in iron steel can form fine particles in scale layers, and iron ions in acid pickling solutions can re-precipitate as iron oxide and iron hydroxide on the surface of steel during water washing and drying. Smut is considered to be composed of such fine particles, iron oxide, and iron hydroxide. Non-uniform material erosion corresponding to the structure of steel can increase the surface roughness. Smut deposition can turn wire rod to a dark color, cause unevenness in the coating treatment, and create an uneven color appearance. It is therefore desired to reduce them.
Non-uniform material erosion, a disadvantage of acid pickling for descaling, should be reduced. For this purpose, many acid pickling inhibitors for reducing the erosion of steel materials are proposed, including nitrogen-containing compounds such as amine compounds, sulfur compounds such as thiourea derivatives, and surfactants. Unfortunately, such inhibitors all have the disadvantage that they can also change the descaling rate, and are not sufficiently effective. On the other hand, smut produced by acid pickling is generally removed using the physical effect of high-pressure water shower, for example, as shown in Patent Literature 1. However, this method uses a relatively large-scale facility or apparatus and also uses a large amount of water. Therefore, this method puts a high load on a wastewater treatment and other processes and also has a problem with cost. There is also the disadvantage that because the removal of smut is less stable, not only the color tone after the acid pickling cannot be kept constant, but also the subsequent coating treatment can be non-uniform, which has an adverse effect on drawing or tertiary processing. It is therefore desired to overcome the disadvantage.
In addition, after the acid pickling, wire rod is exposed to the air in the acid pickling line, so rust can be formed on its surface even while it is transferred to a water washing treatment before a neutralization or coating treatment. To prevent such rusting, for example, Patent Literature 2 discloses a method for preventing the surface of wire rod from rusting by a low-temperature, low-concentration, sulfuric acid treatment that includes immersing the wire rod in a low-temperature, low-concentration, sulfuric acid solution with a concentration of 1 to 10% and at a temperature of 0 to 50° C. after the acid pickling of the wire rod and then washing the wire rod with water, which is performed in a series of wire rod descaling steps including acid pickling, water washing, neutralizing, and coating steps.
Patent Literature 1 discloses a technology about a wire rod-rotating type pressurized water cleaner that is configured to rotate wire rod itself so that it can apply pressurized water almost equally to the whole of the wire rod and also configured to rotate deformed wire rod so that it can wash the wire rod with water while moving the wire rod. As mentioned above, however, this technology has the disadvantage that it puts a high load on the facility and apparatus, particularly, the wastewater treatment system.
According to Patent Literature 1, wire rod is rotated, and therefore the wire rod is washed with water while being moved, so that scale and smut adhering to the surface of coiled wire rod can be removed from the whole circumference of the wire rod. As this water washing becomes more satisfying, the surface of wire rod becomes more active, and the surface becomes rusty when it is exposed to the air during transfer from the water washing to neutralization, so that an undesirable appearance defect, what is called yellowing, is observed.
In a current common acid pickling process for wire rod, therefore, after the high-pressure water washing process disclosed in Patent Literature 1 is completed, the acid pickling line disclosed in Patent Literature 2 may be used to immerse the wire rod in the low-temperature, low-concentration, sulfuric acid solution after the acid pickling and before finish water washing, so that the surface of the wire rod can be prevented from rusting before the water washing and after the coating treatment by the use of the low-temperature, low-concentration, sulfuric acid treatment. However, when used together, both techniques require a large wastewater treatment facility and put a high load on a wastewater treatment system.
In an acid pickling and coating system for coiled wire rod, a cleaning step is commonly performed immediately after an acid pickling step so that the subsequent steps can be facilitated. There are several types of cleaning apparatuses used for such a purpose. Among them, a ring tube-type shower apparatus, which is designed to move its ring tube-shaped shower head along the axis of coiled wire rod while emitting a shower, becomes increasingly used instead of other type of shower apparatuses, because it can produce a high cleaning effect with small pump performance. However, such a shower apparatus still has the problem of rust formation during the shower process.
Patent Literature 3 discloses a system as a measure against rusting during the shower process performed using these shower apparatuses. In such a system, an auxiliary shower for preventing rusting is used in addition to the cleaning shower.